As terrestrial and satellite communication traffic demand and loading increase, physical limits of channel capacity and isolation between channels are stressed. These physical limits constrain network capacity. Network capacity is proportional to channel capacity and the number of channels that can be supported or created in a geographic area. Channel capacity limits the amount of information that can flow over a channel. In all wireless communication networks, radiowave spreading causes interference that limits the utility of channels that can be shared in the same geographic area. As a result, the number of useable channels is constrained, and network capacity is limited.
A typical cellular wireless network connects one or more base stations (BSs) with a highly interconnected mesh network of landlines that distribute message traffic globally and exchange traffic with traditional “wired” telephones as well as Internet networks. Each BS extends this traffic connectivity to a cluster of users (i.e., user equipment (UE) for each user) in a geographically-limited “cell” using wireless radio communications technology. Each cell employs a “star” topology having a BS as the “hub” and radio connectivity to the UEs constituting the “spokes”. All UEs within a cell share the same spectrum, a spectrum that is disjoint from the spectrum shared by UEs in adjacent cells to prevent adjacent cell interference. The size of the network (e.g., the number of users) and the geographic area coverage of the cells, together with the useable UE channel bandwidth and useable UE channel power, are contributors to network capacity. Deploying additional BSs in areas of high population density accommodates a larger number of cellular users, and as a result, revenue for the “carriers”, i.e., the providers of the network, is increased. Licenses to operate cellular networks in highly populated areas are currently valued by the carriers at several dollars per MHz of spectrum for each potential customer occupying the network coverage area.
To minimize interference between wireless networks operating in the same geographic areas (e.g., AT&T®, Verizon®, etc. in some cities) frequency channelization is often employed to allow the different networks to use different frequencies. This work-around is limited by the available bandwidth under the allocation rules and the necessary bandwidth of the individual channels. When networks share bands, additional measures must generally be taken to maximize network capacity. For example, transmit power, as well as geographic separation between network transmitters and receivers, must be managed to provide sufficient isolation to avoid significant loss of capacity to one or both networks due to inter-system interference. Interference, as used herein, is measured in units of power, such as Watts (or as more commonly used among engineers, as 10 times the base-ten logarithm of that power). Isolation of two terminals at the ends of a wireless communication channel is defined herein as the ratio of power received to the power transmitted in the same spectrum. This ratio may be stated as a decimal fraction (or, as more commonly used by engineers, as 10 times the base-10 logarithm of that ratio). While geographic separation contributes to this isolation, obstacles along the radio propagation path and the antenna gains at each end of the channel are physical mechanisms that affect this ratio. In particular, while obstacles generally increase this ratio, antenna gain generally reduces the ratio. The portion of isolation that is due to losses along the radiowave path through the atmosphere is commonly referred to as propagation loss, which excludes antenna gain (or antenna loss) at each end of the channel. In the sequel, path loss and isolation (hereinafter “path loss/isolation”) refer to the same ratio, and propagation losses are assumed to be accounted for as a component of path loss/isolation. UE path loss/isolation from BSs and other radiocommunication stations varies in time due to changes in the geometries of obstacles along the propagation path, as well as changes in the antenna gains resulting from changes in path direction in azimuth and elevation.
Within a single cellular network, interference between UEs sharing a band limits network capacity. For long term evolution (LTE) networks in particular, isolation/path losses between the UEs and the BSs are detected by the BSs, which make changes to UE transmit power allocations within milliseconds to ensure that the UEs can reliably communicate with their own BSs, to minimize inter-cell interference (i.e., with other BSs), and to maximize network capacity. The UE path loss/isolation determined by each BS is noted in periodic UE-to-BS channel quality indications (CQIs), which reflect the positive effect of UE transmit power and the negative effect of path loss between the UEs and the BSs upon the UE signal detectability (limited by the signal-to-noise ratio within the BS receiver).
Under some existing LTE allocation rules, the BS allocates just enough power to UEs to overcome UE-to-BS path loss/isolation (a minimum bound on UE power) in order to preserve battery life. BSs will also sometimes allocate more power to those UEs that have higher channel quality to maximize network capacity. When there are radiocommunication stations requiring legally-enforceable interference protection, the LTE allocation process may require expansion to reflect the additional constraint of “acceptable” aggregate interference. Aggregate interference is the sum of all UE-caused interference powers, measured at the protected station, typically stated in terms of Watts or Milliwatts. Satisfaction of this constraint may require modification of allocations affecting UE operations, i.e., applying upper bounds to UE transmit powers and possibly applying lower bounds to UE proximity to the protected station.
National spectrum management rules require a wireless carrier to obtain a Federal Communication Commission (FCC) license before transmitting on specific frequencies in specific locales currently occupied by National Oceanic and Atmospheric Administration (NOAA) Earth stations, such as Wallops Island, Va., Suitland, Md., Sioux Falls, S. Dak., etc. The path loss/isolation of channels in the licensed area is critical to the total network capacity, and channel isolation becomes problematic when dissimilar services occupy the same geographic area, particularly when one of the services requires a high degree of interference protection.
As a consequence of the recent conclusion of FCC spectrum Auction 97, the FCC is scheduled to issue wireless licenses in bands and geographic areas that were unavailable before and were previously assigned to different services (e.g., satellite services). However, these bands and areas, which have been auctioned, will continue to be used by federal satellite systems, e.g., a U.S. meteorological satellite system operating in the 1695-1710 MHz band whose missions are critical and must be protected from interference from mobile wireless licensees.
The meteorological satellite system includes downlinks to protected radio communications stations, such as an Earth station, a ground station, etc., that are extremely sensitive, i.e., have a very low noise temperature necessary to detect and demodulate a wide-band signal from a small transmitter located in a satellite thousands of miles away. Mobile wireless interference may cause disruption in Earth station downlink signal processing, and cause the loss of downlink information collected by the satellite, impacting critical missions. A receiver at the Earth station may receive an unacceptable level of interference when the UE is approximately 100 kilometers away and when the UE is high enough in altitude (e.g., on a mountain top) to have a clear line of sight to the Earth station. However, due to the terrain between the UE and the Earth station, a direct line of sight may be prevented, and the interference may be significantly reduced. The path loss/isolation thus isolates UEs from the Earth station. This path loss/isolation at any one UE varies with time, by as much as a factor of 10 within a few seconds, affecting the UE interference at the Earth station. When the path loss/isolation is temporarily high, there is an opportunity for some UEs to operate in the vicinity of those Earth stations and at higher power, both of which could contribute to network capacity. Wireless network technology is moving toward exploiting these opportunities to temporarily increase network capacity.
An important advancement in this direction has been the planning for multi-band networks that allow UEs to switch between disparate spectral bands on an opportunistic basis. This strategy capitalizes on the likelihood that at least one of the alternate bands is capable of supporting the UE links at any time (a likelihood that is higher than that in either band, individually). Thus, given the necessity of sharing spectrum and meeting an additional allocation constraint, i.e., aggregate interference received by a protected station, the opportunities for network capacity are increased by multi-band operation.
Current wireless networks employ network optimization software that exercises dynamic UE power control to maximize network capacity. However, these wireless networks are not presently configured to employ in situ measurements of path loss/isolation between all UEs and the Earth station in near-real-time. Consequently, near-real-time path loss/isolation information is not utilized by current wireless networks to allocate UE power to address the scenario above.
Since the Earth station antenna is generally located on the order of 10 meters above the ground, the Earth station typically has a horizon on the order of 10 kilometers. As such, radiation from an interfering UE at distances of 10 kilometers or beyond is often significantly reduced by the interaction with the terrain. This provides some needed isolation between UEs and the Earth station, reducing the received interference power.
When wireless carriers seek licenses for federal bands near NOAA Earth stations, the wireless carriers are subject to prohibition from causing interference exceeding a level acceptable to the incumbent NOAA Earth station, for example. The FCC has adopted a strategy in authorizing wireless licenses that requires a “coordination zone” to be established by network carriers and NOAA outside of which a UE may transmit without specific authorization by NOAA. The coordination zone thus inhibits the UE from transmitting within a specific circle, centered on the Earth station. For example, UEs licensed for the market area surrounding Sioux Falls, S. Dak. may not operate when the UEs are within about 40 kilometers of the Earth station without prior coordination with the Earth station operator. This strategy would be problematic if instituted for several reasons.
First, for the time being, the process for zone radius calculation is based on median values of UE path loss/isolation predicted by a government-approved theoretical model. This means that the “acceptable” level of aggregate interference might be exceeded half the time. Second, excursions of the actual path loss/isolation up to 10 dB from the theoretical median value are to be expected, resulting in large violations of the “acceptable” level of UE interference received by the Earth station, even when the UEs operate well outside of the coordination zone. Third, the theoretical model predictions depend on path loss/isolation measurements made in locations far removed from the sites in question, resulting in significant prediction departures from actual values of path loss/isolation. Lastly, by relying on fixed-values of theoretical path loss/isolation rather than actual path loss/isolation, there are additional network capacity opportunities that are foreclosed by this process. Studies show that the same theoretical model of interference predicts that the UEs might operate as close as 15 km from the Sioux Falls NOAA Earth station at least 50% of the time if the variations in path loss/isolation were exploited, even if the Earth station were receiving downlinks 100% of the time.
In summary, significant opportunistic network capacity exists inside the current coordination zones about the primary protected stations. However, conventional systems cannot realize this capacity because conventional systems fail to detect the temporary high path loss/isolation values that exist.
Depending on the location of the population with respect to the Earth station, there may be conflict between business interests of the wireless carrier operating under, or seeking, a license and the interests of the incumbent operator (e.g., the federal government in the case of the NOAA). For example, the federal government may decide on the specific protection zone, and the zone may include a very populated area, inhibiting the wireless carriers from using the band in an area capable of producing significant revenue for the wireless carrier. The net result is that very lucrative wireless network support of a large number of customers cannot be accommodated inside the protection zones because the wireless networks are not endowed with the ability to act on the opportunity to serve those customers in near-real-time.
When wireless carriers determine whether they should operate or seek a license to operate in the protected user's band, the wireless carriers make assumptions on how much path loss/isolation exists between the operational UE locations and the protected station. Path loss/isolation is highly dependent on the shape of the intervening terrain, and to a lesser extent, the weather. For example, at separation distances of less than 100 kilometers, variation in the path loss/isolation of 5 dB or more may occur over a year's time due to weather and seasonal changes, and short term variations in the path loss/isolation of 5 dB or more can be anticipated as a consequence of rapid movement of the UE over the terrain (e.g., in automobiles on a high speed highway).
In addressing the concerns about wireless network interference to NOAA Earth stations, NOAA investigated monitoring interference inside the Earth station as a method of protection. In this method, when the interference becomes close enough to the “acceptable” level that a violation can be considered likely, the Earth station would send a message to the wireless carrier to reduce the power on the UEs contributing to the (potential) violation of the “acceptable” interference level. This method is problematic for several reasons. First, due to the dynamic variation in path loss/isolation, the Earth station would generally not be able to reliably predict the violation until it was about to occur, leaving too little time for corrective action by the wireless network to prevent the violation. Second, in an attempt to increase the likelihood of prevention, the Earth station may lower its “alarm” threshold, causing an (unavoidable) increase in the false alarm rate, potentially leading the wireless network to incur costs and possibly resist the government's attempts to protect the Earth station by this approach. Third, without accurate UE-to-protected station path loss/isolation information, the wireless network cannot accurately determine which UEs should reduce power and how much reduction would be required to meet the “acceptable” level of aggregate interference. These are fundamental limitations of monitoring as a way to avoid unacceptable interference in a shared band, which lead to large losses of network capacity.
Thus, an alternative approach may be beneficial to prevent UE violation of the “acceptable” interference level of a protected radiocommunication system in a shared band, while permitting the maximum possible opportunistic network capacity attained from less restrictive UE deployment and higher limits on UE transmit power.